Integrated photonics offer the promise of combining many optical devices on a substrate, leading to miniaturization and increased functionality on a chip. Optical devices include but are not limited to optical isolators, frequency converters, amplitude or frequency modulators, couplers, splitters, and combiners. Integrated photonics can be formed from optical devices compatible with commonly used semiconductor fabrication techniques and materials and of a size scale suitable for dense integration.
An optical isolator is a nonreciprocal device that allows transmission of an optical wave in one direction but blocks it in the reverse direction. An optical isolator is analogous to a diode having a low electrical resistance between its input pod and output pod and a very high resistance between its output pod and its input pod, thereby permitting electrical current flow predominantly in one direction. Similarly, a light wave fed into the input pod of an optical isolator is guided with low loss to its output pod and any counter propagating light wave is attenuated so that only a minimal amount of power leaves the optical isolator via the input pod. A reciprocal device is symmetric upon interchange of the input and the output. In a nonreciprocal device, this interchange symmetry does not exist. Optical isolators can be used in optical systems since backscattered light creates noise and instability in laser sources, particularly in integrated platforms where the potential for backscattering increases with device density.
For bulk optical devices, a free-space optical isolator, also known as a bulk isolator, makes use of the Faraday effect, where a magneto-optic material produces a nonreciprocal polarization rotation to achieve isolation. An input field is transmitted through an input polarizer followed by a Faraday rotator, which provides a 45° polarization rotation to the incident beam, and finally an output polarizer oriented at 45° with respect to the input polarizer. In the forward the direction, the field is completely transmitted as it is always polarized along the polarizer axes. In the reverse direction, the beam undergoes an additional 45° polarization rotation that is blocked by the input polarizer.
A dielectric waveguide incorporating the Faraday concept suitable for fabrication using semiconductor industry process steps can be made of a magnetically active material to achieve an integrated waveguide Faraday rotator.
A nonreciprocal phase shift isolator can make use of an interferometer arrangement. Both arms of the interferometer can contain a magneto-optic material with a magnetic field applied in a direction transverse to the propagation direction. In the forward direction, fields in both arms of the interferometer add constructively in phase. In the reverse direction, a nonreciprocal phase shift in one or both arms produces destructive interference and therefore isolation.
In a nonreciprocal loss isolator, a magneto-optic metal produces nonreciprocal loss in the optical wave. An active amplifying medium compensates for the loss in the forward direction. In the reverse direction, the nonreciprocity results in a larger loss resulting in isolation.
Surface plasmon waveguides have been suggested for use in a non-reciprocal phase shift interferometer arrangement. The magneto-optic metals (such as Co) and magneto-optic dielectrics (such as yttrium iron garnet (YIG) and bismuth iron garnet (BIG)), are not compatible with III-V optoelectronic devices and the processes for fabricating them.
An optical waveguide isolator based on a Y-shaped branching waveguide coupler is formed in one layer of a device made from III-V materials (such as GaAs or InP) with one isolating branch integrated with a III-V light emitting diode. With a 2° branching angle, a transmission in the forward direction of 41%, which corresponds to an insertion loss of approximately 3.8 dB, and a transmission in the backward or reverse direction of 0.16%, which corresponds to an isolation of approximately 28 dB, can be achieved. The isolator strength, defined as the ratio of insertion loss to isolation, can be 256. With a 3° branching angle and a cascade of four optical waveguide isolators, an isolator strength of 625 can result. Insertion loss is the loss experienced by an optical wave propagating through an optical isolator in forward direction.
A planar waveguide optical isolator comprising a Y-shaped combiner/splitter can include a planar waveguide N-way splitter/combiner. The forward directed signal is coupled into one of N−1 input waveguides, propagates through a coupling region, and is then coupled into the output waveguide. Reflected signals coupled into the output waveguide propagate through the coupling region and are split between each of the N−1 input waveguides. Except for the input waveguide, each of the N−1 branches is terminated with an isolating element to prevent further propagation of the reflected signal. The cascade arrangement of the waveguide branches is calculated to achieve the desired isolation. For a single stage isolator capable of achieving 3 dB isolation presuming a 50:50 split between the input port and a single reflecting port, a cascaded arrangement of ten such isolators is required to provide approximately 30 dB of isolation.
A single Y-shaped isolator structure can have a length on the order of 2.5 mm. As noted above, the Y-shaped approach can require a cascade of approximately ten isolators to achieve 30 dB isolation and can consequently require a much greater length.
An optical frequency converter is a device that takes an input field at one frequency and produces one or more output fields at one or more different frequencies. This process is therefore nonlinear, as new frequency components not present in the input field are created in the frequency converter. The efficiency of a nonlinear process requires phase matching between the interacting fields. Traditional methods for phase-matching include exploiting crystal birefringence by propagating waves along different axes of anisotropic crystals, periodically inverting crystal orientation to discretely reset the relative phase (so-called quasi-phase-matching (QPM)), or propagating waves in different waveguide modes to exploit waveguide dispersion. The birefringence method is limited to materials and wavelengths with specific properties, and not generally applicable to integrated photonic devices. QPM can be used in non-birefringent materials and, in waveguide geometries, permits conversion over indefinite propagation distances. QPM can equalize the phase velocity of waves of different frequencies propagating in a crystal by discretely inverting the crystal orientation along its length. Fabrication methods for QPM devices, however, require periodic patterning of crystal structures and have only been demonstrated for a small class of ferroelectric materials and semiconductors such as GaAs and ZnSe.
Amplitude modulation is a third important function in optical communications, where one signal (containing information) is transmitted by varying the strength of a second (carrier) signal. One example of a magneto-optic technique for achieving amplitude modulation uses the Faraday effect. An optical field is transmitted through an input polarizer then coupled to a magneto-optic waveguide followed by an output polarizer oriented at an angle with respect to the input polarizer. When a magnetic field is applied to the magneto-optic waveguide in a direction parallel to the propagation direction, the polarization of the optical field rotates. The output beam is therefore attenuated as it passes through the output polarizer. A similar approach for polarization rotation followed by polarization filtering can be accomplished using an electro-optic birefringent material (such as LiNbO3, GaAs).
Interferometric modulators use an electro-optic material in the waveguide, where an electric field induces birefringence that produces a relative change in phase between the arms of the interferometer. When the optical fields in the interferometer arms are recombined, destructive interference occurs as a result of the induced phase shift.
Surface plasmon based modulators have been developed using interferometric configurations. In some cases, the dielectric material can include a thermo-optic polymer whose temperature rises when an electric signal is applied to the surface plasmon metal, resulting in a phase shift in the interferometer arm. The bandwidth in thermo-optic based devices can be low as a result of the slow heating process.
An electro-optic modulator can use a surface plasmon metal placed directly on top of a dielectric waveguide core material. The upper cladding of the surface plasmon waveguide can include an electro-optic material. An electric field causes the dielectric waveguide mode to be reflected or absorbed in the surface plasmon mode resulting in an amplitude modulation.
A magneto-optic surface plasmon modulator has been developed which works by applying a transverse magnetic field to a magneto-optic material bounded by a surface plasmon guiding metal. A free-space optical beam couples into the surface plasmon metal when the phase-matching condition is satisfied.